New and combined sensors for adaptive-rate pacing.

A new potential indication for cardiac pacing is chronotropic incompetence, that is, an inadequate cardiac rate response to exercise and other metabolic demands. Many patients who have been paced for indications such as complete heart block or sick sinus syndrome also have chronotropic incompetence. Such patients are not adequately treated when fitted with a constant rate pacemaker. Adaptive-rate pacemakers increase the pacing rate in proportion to signals derived from a biosensor which is sensitive to exertion and possibly to other metabolic requirements. These pacemakers have proven valuable for patients with overt chronotropic incompetence. However, no single sensor/algorithm is ideal and improvement has been sought by introducing new sensors, adjusting the algorithms by which biosensor signals are converted to the most appropriate pacing rate, or by combining sensors in such a way that a composite biosensor signal is derived which bears a close linear relationship with the appropriate heart rate. An example of a new sensor is the accelerometer, which is sensitive to a fuller range of movements than the piezo crystal. A successful new algorithm is the rate augmentation algorithm for use with minute ventilation, which provides a better initial pacing rate response. A combination of minute ventilation sensed by impedance changes and movement sensed with piezo crystals maintains the rapid response from the piezo crystal and overcomes its lack of proportionality. Another successful new combination of sensors is QT sensing from the evoked ventricular potential and motion sensing with a piezo crystal. As yet, these innovations have not been exhaustively tested and shown to confer clinical benefit but the improvements are such that an advantage can be expected.